A simple model to assess the long term fate of the AMOC

Studies with global climate models over the past 25 years have shown a range of long-term responses of the Atlantic Meridional Overturning Circulation (AMOC), in response to scenarios in which greenhouse gases are increased then eventually stabilised at some value, possibly after temporarily overshooting that value. AMOC responses include stabilisation at weaker than the pre-industrial level, rapid recovery following a period of quasi-steady weak circulation, overshooting to stronger than pre-industrial strength, or tipping to a quasi-permanent weak state. While many of these studies have gained insight into the mechanisms behind their individual model behaviour, no overarching understanding exists of what determines how a particular model will respond.   

We present a simple AMOC model suitable to characterise the different possible long-term responses, and use it as a (partially) unifying framework to show how the different behaviours can arise from a competition between thermal and haline feedbacks. The results are relevant to defining ‘safe mitigation pathways’ that avoid or reduce the risk of AMOC tipping or potentially dangerous overshoots.